1. Field of the Invention
This invention relates generally to print media handling systems and, more particularly, to an endless-loop, vacuum belt, media transport for a hard copy apparatus.
2. Description of Related Art
Automated business machines for producing or reproducing hard copy documents, such as copiers, printers, facsimile machines, document scanners, and the like, are well known commercially.
Ideally, when working with cut sheet printing media (hereinafter also referred to generically as "paper") for a multiple page document, a hard copy device will automatically feed a single sheet of paper and, when operation is finished on the one sheet, e.g., printing a page with ink-jet pens, it is off-loaded while another sheet immediately follows. A continuous flow of paper sheets by automated feeding and positioning without the necessity of manual handling reduces the time required to accomplish the complete operation. The more quickly and accurately the sheet feeding, the faster the operation can be completed, e.g., scanning a multi-page document into a host computer memory. The mechanisms for media sheet feeding are commonly referred to in the art as an automatic document feeder or "ADF."
Belt type document feeders have been adapted to place a document onto a glass scanning bed. One such exemplary system is shown in U.S. Pat. No. 5,342,133 (Canfield), assigned to the common assignee of the present invention. In co-pending U.S. patent application Ser. No. 09/163,098, filed Sep. 29, 1998 by S. O. Rasmussen et al., for a BELT DRIVEN MEDIA HANDLING SYSTEM WITH FEEDBACK CONTROL FOR IMPROVING MEDIA ADVANCE ACCURACY (assignors to the common assignee herein and incorporated herein by reference), a media handling system has an endless belt having a gripping surface which carries a media sheet through a print zone. Improved media advance accuracy is achieved by including closed loop feedback control.
It has also been a long known commercial practice to use a vacuum force distributed across a surface as a holddown. See e.g., U.S. Pat. No. 3,617,127 (1971) to McDuff for a PHOTOGRAPHIC MATERIAL TRANSPORT WITH VACUUM PLATEN. Paper handling vacuum drums have also been commonly practiced. See e.g., U.S. Pat. No. 4,145,040 (first filed in Switzerland in 1975) to Huber for a GRIPPER DRUM.
There are many attendant problems to the use of vacuum platens for printing operations, whether planar type, drum type or endless belt type. In an ink-jet hard copy apparatus (commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employing ink-jet technology for producing hard copy are well-known; the basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions, or as described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988))--hereinafter simply referred to as "printers"--one such problem is the need for maintaining the media as flat as possible, not only to render the highest quality print, but also because of the interaction of the wet ink with the paper. "Ink" generally can be dye-based or pigment-based and uses water or another evaporative solvent as a carrier. When an image to be recorded has high density, a large amount of water is applied to and driven into the medium which in turn swells erratically, causing the printed regions to become wavy, a phenomenon generally known as cockling. To minimize cockle, ink-jet printers sometimes employ heaters. For example, in U.S. Pat. No. 5,510,822 for an INK-JET PRINTER WITH HEATED PRINT ZONE by Vincent et al. (assigned to the common assignee of the present invention and incorporated herein by reference), a printer includes a platen heater assembly as a means of fixing and drying the ink on the paper and a vacuum fan and associated plurality of platen vacuum holds as a means of holding the paper in close contact with the heater plate assembly.
One problem associated with the use of a combination of endless belt and a heater is that distortion of the belt itself occurs because the belt material simultaneously expands in the heated region but is constrained from expanding by adjacent regions. Thus, a belt will distort locally and ripple, which closely resembles the phenomenon of cockling of the paper, a phenomenon sometimes referred to as "potato chipping." The belt ripples interfere with thermal transfer from the heater to the paper and with maintaining the paper flat with respect to the ink-jet writing instruments. The net effects are poor thermal efficiency, print artifacts due to misplaced drops of ink, and uneven drying of the paper with resultant cockle.
Therefore, there is a need for means to eliminate heat-induced distortion of a paper transport belt.